Before the stem of a hip prosthesis can be inserted into the medullary cavity of the proximal femur, the medullary cavity has to be opened, after resection of the head/neck, and then reamed out and shaped to accommodate the stem that is to be inserted. This is done using a rasp, the stem of which is identical to the configuration of the prosthesis stem. The upper end of the rasp corresponds to the resection plane. Extending from the top of this upper end there is a peg whose direction follows that of the head/neck. This configuration permits a rasp instrument to be applied for the purpose of finishing the resection surface (EP-B-166,085); alternatively, a trial joint head can be placed thereon. The same applies to a trial stem. The rasp can also function as a trial stem.
To insert and release the stem, a grip part is provided which can be connected releasably to the stem via coupling parts. The grip part extends generally parallel to the direction of the stem (DE-U-94 07 621) so that hammer blows on its anvil-shaped end drive the stem home or remove it in substantially the longitudinal direction of the stem.
The coupling parts for connecting the stem to the grip part comprise the peg (described above) located at the upper end of the stem, and a bore located on the grip part which receives this peg. Since the peg extends in the head/neck direction, it protrudes obliquely to the side and away from the stem (in relation to the central longitudinal axis of the stem). To ensure that the lower end of the grip part can receive the stem, the grip part is generally angled off in relation to the direction of the rest of the grip part. Thus, most of the grip part extends with a slight lateral offset in relation to the stem, albeit essentially parallel to the longitudinal axis of the stem.
The coupling parts provided in known instruments of this type also comprise means for arresting the peg in the bore to secure the connection between the stem and the grip part. These arresting means are releasable so that the grip part can be detached from the peg. The known arresting means leave something to be desired. Arresting means designed as a screw connection have the disadvantage that the tightening and releasing of the screw takes some time. Also, such screw connections can come loose under the effect of the blows when the stem is being driven home or withdrawn. In arresting means designed as quick-action connectors, the activation means are typically provided in the immediate vicinity of the coupling arrangements. The disadvantage of such arrangements is that they have to be located and activated deep in the wound. This difficulty also arises in the case of a known instrument (DE-C 196 31 984) in which, in order to couple the stem with the grip part, S-shaped members on these coupling parts have to be brought into engagement before their engagement can be secured by means of a sleeve pushed over them. In yet another coupling arrangement (disclosed in a brochure entitled "Das PCA-Huft-Total-System" from the company Howmedica Kiel), a lever engaging the coupling arrangements can be moved from outside the operating wound. Unfortunately, lever movement is very long, creating a substantial disadvantage given the confined space of the operating field. Finally, most of the known instruments of the type described have the disadvantage that their coupling arrangements are complicated and they are therefore difficult to clean.
Needs exist for surgical rasp tools that address the limitations of the existing instruments.